1. Field of the Invention
This invention relates to a developer for a photosensitive resin printing plate comprising a hydrogen-abstracting agent and a process for producing a photosensitive resin printing plate using said developer which printing plate is broadly used as a printing plate for letterpress printing (said production process being referred to as a plate-making process or plate-making in some cases hereinafter).
2. Related Art
A photosensitive resin printing plate is generally obtained by an exposure step in which only the photosensitive layer of a relief portion is cured by a radical polymerization reaction caused by irradiation with an active radiation and a development step subsequent to the exposure step in which development step the uncured resin in other portions than the relief portion is dissolved in and removed with a given washing solution (developer) or swollen with and dispersed in the washing solution and then mechanically removed, thereby allowing only the cured portion to appear as a relief on the surface of the printing plate.
In general, with a photosensitive resin which is cured by a radical polymerization reaction, a vanishment reaction of a radical active species is caused competitively with a propagation reaction of the radical active species, and hence, when the reaction rate is increased to heighten the sensitivity of the resin, it becomes difficult to increase the degree of polymerization which is governed by the concentration of an initiator. In order to obtain a relief suitable for printing, and in order to obtain the necessary sensitivity (reaction rate) for efficiently and simply carrying out the plate-making works, it is necessary for the photosensitive resin to contain a given amount of an initiator. However, there is a limit in degree of polymerization of resin and hence unreacted substance remains even after the resin has been cured, whereby the surface of a printing plate obtained has a tackiness.
The process for producing a photosensitive resin printing plate is generally carried out in four steps of exposure, development, post-exposure and drying, and the post-exposure is a step of further irradiating the printing plate with an active radiation in order to cure the unreacted substance present in and on the cured plate obtained after the development to thereby increase the mechanical strength of the printing plate and reduce the surface tackiness of the printing plate. However, even the printing plate after the post-exposure step is not necessarily sufficiently free from the surface tackiness.
When a printing plate having a great tackiness is used in printing, there are such problems as poor workability in attaching the plate to and detaching the plate from a plate cylinder, adhesion between plates to each other or attachment of dust to the plates during storing the plates, adhesion of paper powder to the plate during the printing, paper-picking during the printing and the like. In particular, when paper powder adhesion and paper-picking are caused owing to the tackiness of the plate during the printing, it is necessary to discontinue the printing and clean the plate, so that the working efficiency is remarkably decreased.
As a method of solving the above problems, there have been adopted a method in which the photosensitive resin plate is irradiated with an active radiation while the plate is immersed in a liquid during the post-exposure step (a post-exposure-in-water method), a method in which the post-exposure is effected in an inert gas to accelerate the polymerization reaction on the surface of the photosensitive resin layer, a method in which a coating material such as various rubber latexes, saran latex or the like is coated on the photosensitive resin plate to lower the surface tackiness, a method in which an oxidizing agent and a reducing agent are used alone or in admixture to subject the photosensitive resin plate to chemical treatment, a method in which the above methods are combined, and the like. As a kind of coating method, there is proposed a method of removing the surface tackiness in which such a phenomenon is utilized that when a specific compound having a higher alkyl group is previously incorporated into the photo-sensitive resin composition the compound is deposited as crystals on the surface of the printing plate after the plate-making (JP-A-61-120,142).
However, these methods are insufficient in the effect per se, or though in some cases they have any effect, the handling of reagents used becomes a problem. Specifically, (1) the coating method is disadvantageous in that the coating layer is peeled off during the printing, and is insufficient in maintenance of the effect. (2) With some kinds of photosensitive resin compositions, even when the post-exposure is effected in an atmosphere from which oxygen is intercepted such as in a liquid or in an inert gas (referred to hereinafter as the poor oxygen post-exposure), the tackiness removal is insufficient in some cases. (3) In the treatment method using an oxidizing agent and a reducing agent, in general, it is necessary to pay attention to the handling of reagents used and to subject the apparatus contacting the treating solution to rust prevention processing.
In particular, the poor oxygen post-exposure (2) is a method intending that by intercepting oxygen and efficiently carrying out the radical polymerization of the polymerizable double bond in the composition, the resulting prepolymer is three-dimensionally cross-linked, thereby increasing the mechanical strength and simultaneously therewith diminishing the unreacted (namely uncross-linked) prepolymer as much as possible to reduce the tackiness. However, when the photosensitive resin composition contains a large amount of such a prepolymer that the number of polymerizable double bonds in one molecule is 2 or less, the printing plate obtained from the photosensitive resin composition can have mechanical strength increased by the poor oxygen post-exposure, but shows insufficient tackiness-removing effect, and when the photosensitive resin composition contains a large amount of such a prepolymer that the number of polymerizable double bonds is more than 2, it is necessary for the printing plate obtained therefrom to be irradiated with an active radiation for a very long time to achieve a sufficient tackiness removing effect.
That is to say, in the plate-making process used at present, it is difficult to allow all the terminal double bonds of the prepolymer to react and even if all of them should have reacted, a plate quite free from tackiness would not be obtained in many cases. Accordingly, in order to more sufficiently remove the tackiness of the photosensitive resin composition containing such a prepolymer, it is effective to three-dimensionally cross-link the prepolymer by utilizing the reaction of the double bonds and other means together.
As the three-dimensional cross-linking based on other reactions than the reaction of double bonds, JP-B-56-16,182 discloses a method which comprises abstracting hydrogen from the main chain of a prepolymer which has reacted with another prepolymer or the unreacted monomer at the terminal double bonds, thereby producing a radical of the prepolymer, and then cross-linking the main chains of the prepolymers with one another through the reaction of the said radicals. Specifically, the surface layer of a radically polymerizable resin cured product is impregnated with a hydrogen-abstracting agent which can abstract a hydrogen atom from a compound upon irradiation with an active radiation and thereafter the impregnated portion of the cured product is irradiated with an active radiation having a wavelength of 300 nm or less, preferably 200 to 300 nm, thereby removing the surface tackiness of the radically polymerizable resin cured product.
JP-B-56-16,182 illustrates, as specific examples of impregnating the surface layer of the radically polymerizable resin cured product with a hydrogen-abstracting agent, a method comprising dissolving of the hydrogen-abstracting agent in ethanol, isopropanol or a water/ethanol (1/4) mixture and immersing the cured product in the resulting solution or spraying the surface layer of the cured product with the said solution by means of a sprayer.
However, when the cured product is impregnated with a hydrogen-abstracting agent by the above-mentioned method, the surface tackiness removal effect is sufficiently recognized, but it follows that a separate step of impregnating the plate produced by conventional process with the hydrogen-abstracting agent is required, whereby the number of steps for making a plate is increased and the plate-making process is complicated, to thereby decrease the production yield and require a new equipment, resulting in an increase of the plate-making cost.
Moreover, an organic solvent such as alcohols has a low flash point and hence in the above-mentioned method, the handling of the organic solvent is difficult and such an expensive equipment as an explosion proof type dryer, a forced exhauster or the like becomes necessary.